From the DMG: It's easy to overlook the effects of terrain when building adventures and encounters. After all, the party's enemies are the monsters, not the dungeon stairs, the low rock wall, or the crumbled statues in the dungeon room. Yet, terrain provides the context for an encounter. A mob of goblin archers is easy to defeat when only empty terrain lies between it and the party. Take the same goblins, put them on the opposite side of a wide chasm -- or hidden within a grove, or behind a crumbling wall -- and the characters face a much tougher challenge.